Overlay Modulation of COFDM Using Phase and Amplitude Offset Carriers

ABSTRACT

Systems and methods are presented for transmitting additional data over preexisting differential COFDM signals by modulating existing data carriers with a phase and an amplitude offset. In exemplary embodiments of the present invention, additional data capacity can be achieved for an COFDM signal which is completely backwards compatible with existing satellite broadcast communications systems. In exemplary embodiments of the present invention additional information can be overlayed on an existing signal as a combination of amplitude and phase offset from the original QPSK symbols, applied for each information bit of the overlay data. With two additional levels of modulation, a receiver can demodulate the information from each of the previous stages and combine the information into a suitable format for soft decoding. The first stage of demodulation will be recovery of overlay data from the amplitude modulated D8PSK. Because other amplitude variations due to multi-path are also expected, the data gathered from the FFT in the receiver must be equalized to the channel conditions. After channel equalization has been performed, soft overlay data can then be derived from the distance off the unit circle. In order to recover the phase modulated overlay data, the equalized symbols must first be differentially demodulated and corrected for any common phase error offset. After common phase removal, overlay phase information can be obtained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and hereby incorporates byreference U.S. Provisional Patent Application No. 61/072,637 entitled “OVERLAY MODULATION OF COFDM USING PHASE AND AMPLITUDE OFFSET CARRIERS”,filed on Mar. 31, 2008.

TECHNICAL FIELD

The present invention relates to satellite broadcast communications, andmore particularly to methods and systems for transmitting additionaldata over preexisting differential Coded Orthogonal Frequency DivisionMultiplexing (COFDM) signals by modulating existing data carriers with aphase and amplitude offset.

BACKGROUND INFORMATION

Existing satellite broadcast communication systems, such as, forexample, the one currently utilized by Sirius Satellite Radio, employtwo forms of modulation to convey information, single carrier QuadraturePhase Shift Keying (QPSK) and multicarrier differential COFDM.

QPSK is a modulation technique that allows for the transmission ofdigital information across an analog channel. In QPSK, data bits aregrouped into pairs with each pair represented by a particular waveform,commonly referred to as a symbol. There are four possible combinationsof data bits in a pair, and a unique symbol is required for eachpossible combination of data bits in a pair. QPSK creates four differentsymbols, one for each pair, by changing the I gain and Q gain for therespective cosine and sine modulators. The symbol is then sent across ananalog channel after modulating a single carrier. A receiver candemodulate the signal and look at the recovered symbol to determinewhich combination of data bits was sent in an original pair.

COFDM, or Coded Orthogonal Frequency-division Multiplexing (COFDM) is afrequency-division multiplexing (FDM) scheme utilized as a digitalmulti-carrier modulation method. A large number of closely-spacedorthogonal sub-carriers are used to carry data. The data is divided intoseveral parallel data streams or channels, one for each sub-carrier.Each sub-carrier is modulated with a conventional modulation scheme(such as, for example, quadrature amplitude modulation (QAM) or phaseshift keying (QPSK)) at a low symbol rate, maintaining total data ratessimilar to conventional single-carrier modulation schemes in the samebandwidth. For example, a COFDM system can distribute a single digitalsignal across 1,000 or more signal carriers simultaneously. Coded datais modulated and inserted into orthogonal carriers in the frequencydomain. Because signals are sent at right angles to each other, thesignals do not interfere with one another.

One problem that occurs in all RF transmission is multi-path effects.This refers to the scattering of a signal due to obstructions such ascanyons, buildings, etc., that can cause a signal to take two or morepaths to reach its final destination. COFDM is highly resistant tomulti-path effects because it uses multiple carriers to transmit thesame signal, making it a robust transmission method. However, thecurrent modulation techniques used by satellite broadcast communicationsystems, cannot convey additional information overlaid on an COFDMsignal. As overlay modulation, or multi-layer modulation is a useful andefficient method to optimize available bandwidths, the ability tooverlay COFDM signals with multiple layers of modulation is highlydesirable. In systems where overlay modulation is contemplated thatinclude a COFDM transmission in addition to, for example, othertransmissions, such as Time Division Multiplexing (single carrier)transmissions, it would be very useful to be able to implement theoverlay technique on COFDM as well, so that the entire system cansupport overlay modulation.

What is thus needed in the art is an alternative implementation of COFDMthat can overcome or ameliorate the problems of the prior art.

SUMMARY OF THE INVENTION

Systems and methods are presented for transmitting additional data overpreexisting differential COFDM signals by modulating existing datacarriers with a phase and an amplitude offset. In exemplary embodimentsof the present invention, additional data capacity can be achieved foran COFDM signal which is completely backwards compatible with existingsatellite broadcast communications systems. In exemplary embodiments ofthe present invention additional information can be overlayed on anexisting signal as a combination of amplitude and phase offset from theoriginal QPSK symbols, applied for each information bit of the overlaydata. With two additional levels of modulation, a receiver candemodulate the information from each of the previous stages and combinethe information into a suitable format for soft decoding. The firststage of demodulation will be recovery of overlay data from theamplitude modulated D8PSK. Because other amplitude variations due tomulti-path are also expected, the data gathered from the FFT in thereceiver must be equalized to the channel conditions. After channelequalization has been performed, soft overlay data can then be derivedfrom the distance off the unit circle. In order to recover the phasemodulated overlay data, the equalized symbols must first bedifferentially demodulated and corrected for any common phase erroroffset. After common phase removal, overlay phase information can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the basic structure of an exemplary COFDM encoderaccording to an exemplary embodiment of the present invention;

FIG. 2 illustrates a constellation resulting from exemplary QPSKmodulation according to an exemplary embodiment of the presentinvention;

FIG. 3 illustrates a constellation resulting from exemplary differentialQPSK modulation according to an exemplary embodiment of the presentinvention;

FIG. 4 illustrates the basic structure of a COFDM receiver according toan exemplary embodiment of the present invention;

FIG. 5 illustrates a received QPSK signal at modulated carries in amulti-path environment according to an exemplary embodiment of thepresent invention;

FIG. 6 illustrates a constellation resulting from phase modulatedoverlay QPSK according to an exemplary embodiment of the presentinvention;

FIG. 7 illustrates a constellation resulting from differential phasemodulated overlay QPSK according to an exemplary embodiment of thepresent invention;

FIG. 8 illustrates a received QPSK signal at phase overlay modulatedcarriers in a multi-path environment according to an exemplaryembodiment of the present invention;

FIG. 9 illustrates a constellation resulting from amplitude overlaymodulation of differential QPSK with phase overlay according to anexemplary embodiment of the present invention;

FIG. 10 illustrates amplitude modulated overlay recovery according to anexemplary embodiment of the present invention; and

FIG. 11 illustrates phase modulated overlay recovery according to anexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In exemplary embodiments of the present invention, additional data canbe transmitted over preexisting differential COFDM signals by changingthe amplitude and phase of data symbols.

The following systems and methods are proposed for, but not confined to,use in the Sirius Satellite Radio Service Terrestrial Network, acomponent of Sirius' Sirius Satellite Digital Audio Radio System(“SDARS”). In general, the systems and methods of exemplary embodimentsof the present invention can be used in any COFDM system.

As discussed above, in COFDM, coded data is modulated and inserted intoorthogonal carriers in the frequency domain. A time waveform is thencreated by taking an inverse Fast Fourier Transform (FFT) of themodulated carriers and appending a cyclic prefix of the result, known asthe guard interval. FIG. 1 illustrates the basic structure of a COFDMencoder. Carrier modulation is QPSK followed by a π/4 differentialmodulator, resulting in two bits of information per carrier. Use of thedifferential modulator places all of the information in the phasedifference between carriers. FIG. 2 illustrates a constellation diagramof the QPSK symbols while FIG. 3 illustrates a constellation diagram ofthe differential QPSK symbols.

A receiver for the modulation scheme described above recovers theoriginal data by synchronizing to the transmitted waveform, performingan FFT on the appropriate section of data, differential demodulating thedata carriers, correcting for the common phase offset and extractinginformation from the resulting QPSK constellation for soft decoding.FIG. 4 illustrates the basic structure of a COFDM receiver. Due tomulti-path conditions, expected in an environment where COFDM would beused, the lack of channel equalization can result in a received QPSKconstellation as illustrated in FIG. 5. Notable is the elongatedconstellation due to the amplitude variations induced by the multi-pathenvironment. Additionally, the petal-like structure of the receivedconstellation will tend to become wider as the signal-to-noise ratio(SNR) is decreased.

To transmit additional data over the current system without changing theexisting system constraints, data must be overlaid onto the originalinformation. If the original data symbols are modulated with theadditional information, a hierarchical system is created. In exemplaryembodiments of the present invention, the additional information can beat a rate of 1 bit/symbol. To convey the additional information, themodulation scheme according to exemplary embodiments of the presentinvention can, for example, utilize a combination of amplitude and phaseoffset from the original QPSK symbols, applied for each information bitof the overlay data. Each of the modulation functions can, for example,either send the same information, or separate information at twice therate. If the same information is used for example, the data can bemerged at the receiver for a combining gain and will permit the use of ahigher coding rate on the data. If separate information is used, forexample, twice as much data can be sent but a lower coding rate will beneeded making the overall throughput similar to using the sameinformation. In exemplary embodiments of the present invention,identical information can be used in order to take advantage of thecombining gain.

In exemplary embodiments of the present invention a modulation schemecan be separated into two stages. A first stage, for example, can applya fixed offset angle to the original QPSK prior to differentialmodulation and thus create a signal similar to one that would resultfrom 8 Phase Shift Keying (8-PSK). The information bit would dictate thedirection of the applied offset. An equation to apply such a phaseoffset to an original QPSK signal is as follows:

Ovly1I=Cos(α)*SymI−Z*Sin(α)*SymQ

Ovly1Q=Cos(α)*SymQ+Z*Sin(α)*SymI

Where:

-   -   SymI and SymQ are from the original QPSK symbol;    -   Z is the information bit represented as +/−1;    -   α=the modulation angle; and    -   Ovly1I and Ovly1Q form the resulting overlay symbol pair.

It is noted that because Z is +/−1, it controls the direction of thephase rotation by changing the sign of the mixing function. The degreeof offset (α) can be, for example, programmable to any angle between 0and 45 degrees, thus preserving the original quadrant information.However, in exemplary embodiments of the present invention, this anglecan preferably be kept small to minimize the performance degradation ofthe existing system. In one exemplary embodiment, a maximum allowableangle can be, for example, 22.5 degrees, where the symbols would all beof equal distance. FIG. 6 illustrates an exemplary phase modulated QPSKsignal while FIG. 7 illustrates an exemplary differentially modulatedresult of the signal, hereinafter referred to as Differential 8-aryPhase Shift Keying (D8PSK). Because the magnitude of the originalsymbols is maintained, the average power of the overlay symbols is thesame as before.

To legacy receivers in a multi-path environment, the phase modulationwould cause the formation of a pair of petals, as is illustrated in FIG.8. As the SNR is decreased, the petals will become less and lessdistinguishable. Because the original points now start off closer to theadjacent quadrants, errors can begin to occur much sooner than if justthe original QPSK signal was sent. If the impact on the performance ofcurrent receivers were expected to be minimal, the performance of theoverlay data would be greatly restricted, because the maximum permittedoffset angle would be reduced.

After the offset angle is applied to the original QPSK signal, thesymbols can then, for example, be differentially modulated as in theoriginal system. The differential modulation places all information intothe phase between the carriers, thus allowing for additional informationto be carried in the amplitude of the signal. At this point, the secondphase of the modulation can, for example, be introduced.

The overlay data can be interleaved in frequency to add diversitybetween the two overlay modulations. In a multi-path environment, thefrequency diversity can help to provide the best combining gain betweenthe two sets of overlay data. Using the interleaved overlay data, theD8PSK signal can, for example, be further modulated in amplitude. Forexample, the original signal amplitude can be offset by some delta andthe information bit can control the sign. FIG. 9 illustrates theapplication of an amplitude offset to previously phase modulated D8PSKsymbols. Exemplary equations to modify the D8PSK symbols can be asfollows:

Ovly2I=D8pskI*√{square root over (A ^((Z)))}

Ovly2Q=D8pskQ*√{square root over (A ^((Z)))}

Where:

-   -   D8pskI and D8pskQ are the differentially modulated symbol pair;    -   A is the desired power offset;    -   Z is the overlay data represented as +/−1; and    -   Ovly2I and Ovly2Q form the resultant symbol pair.

In exemplary embodiments of the present invention the scaling of theamplitude can be such that the average power will remain the same. Thechoice of A can be programmable but can, for example, be limited to arange (1≦A≦K), where K is chosen suitable to the expected number systemof the receivers.

As noted above, amplitude variations due to multi-path conditions canappear as the elongated signal constellations as illustrated in FIG. 5.Therefore, the amplitude variations induced by the overlay modulationwould appear as multi-path to existing receivers. Because the data isexpected to look random, the effects at the receiver should average out.That is, the overlay symbols will appear as constructive interferencehalf of the time while they will appear as destructive interference theremainder of the time. One of the key advantages of this part of themodulation is the minimal impact on existing receivers. As discussed ingreater detail below, recovery of the data will require additionalcomplexity in the new receiver.

With two additional levels of modulation, a receiver must be designed todemodulate the information from each of the previous stages and combinethe information into a suitable format for soft decoding. The firststage of demodulation will be recovery of overlay data from theamplitude modulated D8PSK. Because other amplitude variations due tomulti-path are also expected, the data gathered from the FFT in thereceiver must be equalized to the channel conditions. Copending U.S.patent application Ser. No. 12/184,659, under common assignmentherewith, entitled OVERLAY MODULATION TECHNIQUE FOR COFDM SIGNALS BASEDON AMPLITUDE OFFSETS, filed on Aug. 1, 2008, hereby incorporated byreference in its entirety, describes in detail how to implement channelequalization required for extracting overlay data with a notablecomplexity. After channel equalization has been performed, the resultingconstellation should resemble the rings as illustrated in FIG. 9, exceptnormalized for unity magnitude and ring widths dependant on SNR. Thesoft overlay data can then be derived from the distance off the unitcircle, as illustrated in FIG. 10. This value must be saved for latercombination with data recovered from the phase modulation.

In order to recover the phase modulated overlay data, the equalizedsymbols must first be differentially demodulated and corrected for anycommon phase error offset. After common phase removal, the resultantconstellation should resemble the constellation as illustrated in FIG.6, except noise will form clouds around the original symbol locations.The overlay information is contained in the delta from the 45 degreeaxis within each quadrant. To simplify the measurements, all the symbolscan be mapped back to the first quadrant by taking the absolute value ofthe symbols. The soft symbol value for the overlay data can then betaken as the delta from the y=x axis, as illustrated in FIG. 11.

If the information used to apply the amplitude and phase offset was thesame, the results must be combined. The simplest form of combining wouldbe adding the two results together. The combined signal would then bepassed as a soft decoding value to a Forward Error Correction block toextract the original data sequence. Use of phase modulation by itself islimited due to the impact on current receivers, which will decrease inperformance as the modulation angle is increased. The differentialdemodulation will also have a negative impact to the performance becausedifferential modulation puts all the information in the phase differencebetween the carriers. Amplitude modulation avoids the loss due todifferential demodulation but is limited in range of values. Even thoughthe amplitude modulation algorithm keeps the average power constant, theamount of separation between the overlay symbols is still limited by thefinite precision of the receivers. Together, the combined performance ofamplitude and phase modulation offer a worthy approach to overlaymodulation in COFDM systems and overcomes problems in the prior art.

In exemplary embodiments of the present invention, the disclosed systemsand methods can be implemented in hardware or software, or anycombination thereof, both specialized or otherwise. In exemplaryembodiments of the present invention, the disclosed systems and methodscan be implemented in one or more ASICs, or FPGAs, or the like, or inspecialized systems designed to broadcast and receive modulated RFsignals. In exemplary embodiments of the present invention, receiversusing the disclosed systems and methods can be implemented in areceiver, such as for example, one of the various types of satelliteradio receivers provided or licensed by Sirius XM Radio, Inc. Suchreceivers generally have one or more baseband chips that containspecialized hardware and/or software for demodulating and decoding areceived satellite radio signal.

Similarly, in exemplary embodiments of the present invention,transmission systems using the disclosed systems and methods can beimplemented in a transmitter complex, such as, for example, one of thevarious types of satellite radio transmitters utilized in generating andtransmitting one of the Sirius XM Radio, Inc. signals.

While the present invention has been described with reference to certainexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from its scope.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed, but that the invention will include allembodiments falling within the scope of the appended claims.

1. A method of transmitting additional data over a preexisting signal,comprising: providing a first modulated signal; overlaying additionalinformation on the first signal by changing the amplitude and phase ofdata symbols of said first modulated signal to create an output signal,wherein the first signal is a differential COFDM signal.
 2. The methodof claim 1, wherein said changing comprises adding a combination ofamplitude and phase offsets from the original QPSK symbols, applied foreach information bit of the overlay data.
 3. The method of claim 1,wherein a first overlay stage applies a fixed offset angle to theoriginal QPSK prior to differential modulation to create a signalsimilar to one that would result from 8 Phase Shift Keying (8-PSK). 4.The method of claim 1, wherein the following equation is used to apply aphase offset to an original QPSK signal:Ovly1I=Cos(α)*SymI−Z*Sin(α)*SymQOvly1Q=Cos(α)*SymQ+Z*Sin(α)*SymI where SymI and SymQ are from theoriginal QPSK symbol; Z is the information bit represented as +/−1;α=the modulation angle; and Ovly1I and Ovly1Q form the resulting overlaysymbol pair.
 5. The method of claim 4, wherein after said phase offsethas been applied, the resulting symbols can be further modified asfollows:Ovly2I=D8pskI*√{square root over (A ^((Z)))}Ovly2Q=D8pskQ*√{square root over (A ^((Z)))} where: D8pskI and D8 pskQare the differentially modulated symbol pair; A is the desired poweroffset; Z is the overlay data represented as +/−1; and Ovly2I and Ovly2Qform the resultant symbol pair.
 6. The method of claim 1, wherein atleast one of (i): scaling of the amplitude can be such that the averagepower will remain the same, and (ii) said amplitude offset is appliedafter application of a phase offset and a differential modulationprocess.
 7. The method of claim 5, wherein A is one or more of (i)programmable, and limited to a range (1≦A≦K), where K is chosen suitableto the expected number system of the receivers.
 8. (canceled)
 9. Themethod of claim 1, wherein the overlay amplitude offset data can bedecoded by computing the vector distance of each point and comparingagainst a center decision ring.
 10. (canceled)
 11. A data processor,tangibly embodying a program of instructions executable by the dataprocessor to implement a method of transmitting additional data over apreexisting signal, said method comprising: providing a first modulatedsignal; overlaying additional information on the first signal bychanging the amplitude and phase of data symbols of said first modulatedsignal to create an output signal, wherein the first signal is adifferential COFDM signal.
 12. The data processor claim 11, wherein saidchanging comprises adding a combination of amplitude and phase offsetsfrom the original QPSK symbols, applied for each information bit of theoverlay data.
 13. The data processor of claim 11, wherein a firstoverlay stage applies a fixed offset angle to the original QPSK prior todifferential modulation to create a signal similar to one that wouldresult from 8 Phase Shift Keying (8-PSK).
 14. The data processor ofclaim 11, wherein the following equation is used to apply a phase offsetto an original QPSK signal:Ovly1I=Cos(α)*SymI−Z*Sin(α)*SymQOvly1Q=Cos(α)*SymQ+Z*Sin(α)*SymI where SymI and SymQ are from theoriginal QPSK symbol; Z is the information bit represented as +/−1;α=the modulation angle; and Ovly1I and Ovly1Q form the resulting overlaysymbol pair.
 15. The data processor of claim 14, wherein after saidphase offset has been applied, the resulting symbols can be furthermodified as follows:Ovly1I=D8pskI*√{square root over (A ^((Z)))}Ovly2Q=D8pskQ*√{square root over (A ^((Z)))} where: D8pskI and D8pskQare the differentially modulated symbol pair; A is the desired poweroffset; Z is the overlay data represented as +/−1; and Ovly2I and Ovly2Qform the resultant symbol pair.
 16. The data processor of claim 11,wherein at least one of (i): scaling of the amplitude can be such thatthe average power will remain the same, and (ii) said amplitude offsetis applied after application of a phase offset and a differentialmodulation process.
 17. The data processor of claim 15, wherein A is oneor more of (i), and (ii) limited to a range (1≦A≦K), where K is chosensuitable to the expected number system of the receivers.
 18. (canceled)19. The data processor of claim 11, wherein the overlay amplitude offsetdata can be decoded by computing the vector distance of each point andcomparing against a center decision ring.
 20. (canceled)
 21. The dataprocessor of claim 11, wherein said data processor is one of an ASIC, acomponent of a baseband chip, and an FPGA.
 22. A receiver, comprising: afirst demodulation stage; a channel equalizer; and a second demodulationstage, wherein the first stage recovers overlay data from amplitudemodulated phase offset symbols, and wherein the second demodulationstage differentially demodulates the equalized symbols, corrects for anycommon phase error offset, and determines phase overlay offset.
 23. Themethod of claim 1, wherein to recover the overlay data at a receiver,channel amplitude equalization is used to extract said data.